Book 1, Chapter 13: The unity of the Divine Essence in three Persons taught, in Scripture, from the foundation of the world.
Section 16. What view to be taken of the Trinity. The form of Christian baptism proves that there are three persons in one essence. The Arian and Macedonian heresies.
Section 17. Of the distinction of Persons. They are distinct, but not divided. This proved.
Section 18. Analogies taken from human affairs to be cautiously used. Due regard to be paid to those mentioned by Scripture.
Section 19. How the Three Persons not only do not destroy, but constitute the most perfect unity.
baptism proves that there are three persons in one essence. The Arian and Macedonian heresies.
But as God has manifested himself more clearly by the advent of Christ, so he has made himself more familiarly known in three persons. Of many proofs let this one suffice. Paul connects together these three, God, Faith, and Baptism, and reasons from the one to the other, viz., because there is one faith he infers that there is one God; and because there is one baptism he infers that there is one faith. Therefore, if by baptism we are initiated into the faith and worship of one God, we must of necessity believe that he into whose name we are baptised is the true God. And there cannot be a doubt that our Saviour wished to testify, by a solemn rehearsal, that the perfect light of faith is now exhibited, when he said, “Go and teach all nations, baptising them in the name of the Father, and of the Son, and of the Holy Spirit,” (Mat 28: 19) since this is the same thing as to be baptised into the name of the one God, who has been fully manifested in the Father, the Son, and the Spirit. Hence it plainly appears, that the three persons, in whom alone God is known, subsist in the Divine essence. And since faith certainly ought not to look hither and thither, or run up and down after various objects, but to look, refer, and cleave to God alone, it is obvious that were there various kinds of faith, there behaved also to be various gods. Then, as the baptism of faith is a sacrament, its unity assures us of the unity of God. Hence also it is proved that it is lawful only to be baptised into one God, because we make a profession of faith in him in whose name we are baptised. What, then, is our Saviour’s meaning in commanding baptism to be administered in the name of the Father, and the Son, and the Holy Spirit, if it be not that we are to believe with one faith in the name of the Father, and the Son, and the Holy Spirit? But is this any thing else than to declare that the Father, Son, and Spirit, are one God? Wherefore, since it must be held certain that there is one God, not more than one, we conclude that the Word and Spirit are of the very essence of God. Nothing could be more stupid than the trifling of the Arians, who, while acknowledging the divinity of the Son, denied his divine essence. Equally extravagant were the ravings of the Macedonians, who insisted that by the Spirit were only meant the gifts of grace poured out upon men. For as wisdom understanding, prudence, fortitude, and the fear of the Lord, proceed from the Spirit, so he is the one Spirit of wisdom, prudence, fortitude, and piety. He is not divided according to the distribution of his gifts, but, as the Apostle assures us, (1Co 12: 11) however they be divided, he remains one and the same.
On the other hand, the Scriptures demonstrate that there is some distinction between the Father and the Word, the Word and the Spirit; but the magnitude of the mystery reminds us of the great reverence and soberness which ought to he employed in discussing it. It seems to me, that nothing can be more admirable than the words of Gregory Nanzianzen: “Ou ftano to ei noesai, kai tois trisi perilampomai; ou ftavo ta tria dielein kai eis to hen anaferomai”, (Greg. Nanzian. in Serm. de Sacro Baptis.) “I cannot think of the unity without being irradiated by the Trinity: I cannot distinguish between the Trinity without being carried up to the unity.” Therefore, let us beware of imagining such a Trinity of persons as will distract our thoughts, instead of bringing them instantly back to the unity. The words Father, Son, and Holy Spirit, certainly indicate a real distinction, not allowing us to suppose that they are merely epithets by which God is variously designated from his works. Still they indicate distinction only, not division. The passages we have already quoted show that the Son has a distinct subsistence from the Father, because the Word could not have been with God unless he were distinct from the Father; nor but for this could he have had his glory with the Father. In like manner, Christ distinguishes the Father from himself when he says that there is another who bears witness of him, (John 5: 32; 8: 16) To the same effect is it elsewhere said, that the Father made all things by the Word. This could not be, if he were not in some respect distinct from him. Besides, it was not the Father that descended to the earth, but he who came forth from the Father; nor was it the Father that died and rose again, but he whom the Father had sent. This distinction did not take its beginning at the incarnation: for it is clear that the only begotten Son previously existed in the bosom of the Father, (John 1: 18) For who will dare to affirm that the Son entered his Father’s bosom for the first time, when he came down from heaven to assume human nature? Therefore, he was previously in the bosom of the Father, and had his glory with the Father. Christ intimates the distinction between the Holy Spirit and the Father, when he says that the Spirit proceedeth from the Father, and between the Holy Spirit and himself, when he speaks of him as another as he does when he declares that he will send another Comforter; and in many other passages besides, (John 14: 6; 15: 26; 14: 16).
I am not sure whether it is expedient to borrow analogies from human affairs to express the nature of this distinction. The ancient fathers sometimes do so, but they at the same time admits that what they bring forward as analogous is very widely different. And hence it is that I have a great dread of any thing like presumption here, lest some rash saying may furnish an occasion of calumny to the malicious, or of delusion to the unlearned. It were unbecoming, however, to say nothing of a distinction which we observe that the Scriptures have pointed out. This distinction is, that to the Father is attributed the beginning of action, the fountain and source of all things; to the Son, wisdom, counsel, and arrangement in action, while the energy and efficacy of action is assigned to the Spirit. Moreover, though the eternity of the Father is also the eternity of the Son and Spirit, since God never could be without his own wisdom and energy; and though in eternity there can be no room for first or last, still the distinction of order is not unmeaning or superfluous, the Father being considered first, next the Son from him, and then the Spirit from both. For the mind of every man naturally inclines to consider, first, God, secondly, the wisdom emerging from him, and, lastly, the energy by which he executes the purposes of his counsel. For this reason, the Son is said to be of the Father only; the Spirit of both the Father and the Son. This is done in many passages, but in none more clearly than in the eighth chapter to the Romans, where the same Spirit is called indiscriminately the Spirit of Christ, and the Spirit of him who raised up Christ from the dead. And not improperly. For Peter also testifies (1Pe 1: 11) that it was the Spirit of Christ which inspired the prophets, though the Scriptures so often say that it was the Spirit of God the Father.
Moreover, this distinction is so far from interfering with the most perfect unity of God, that the Son may thereby be proved to be one God with the Father, inasmuch as he constitutes one Spirit with him, and that the Spirit is not different from the Father and the Son, inasmuch as he is the Spirit of the Father and the Son. In each hypostasis the whole nature is understood the only difference being that each has his own peculiar subsistence. The whole Father is in the Son, and the whole Son in the Father, as the Son himself also declares, (John 14: 10) “I am in the Father, and the Father in me;” nor do ecclesiastical writers admit that the one is separated from the other by any difference of essence. “By those names which denote distinctions” says Augustine “is meant the relation which they mutually bear to each other, not the very substance by which they are one.” In this way, the sentiments of the Fathers, which might sometimes appear to be at variance with each other, are to be reconciled. At one time they teach that the Father is the beginning of the Son, at another they assert that the Son has both divinity and essence from himself, and therefore is one beginning with the Father. The cause of this discrepancy is well and clearly explained by Augustine, when he says, “Christ, as to himself, is called God, as to the Father he is called Son.” And again, “The Father, as to himself, is called God, as to the Son he is called Father. He who, as to the Son, is called Father, is not Son; and he who, as to himself, is called Father, and he who, as to himself, is called Son, is the same God.” Therefore, when we speak of the Son simply, without reference to the Father, we truly and properly affirm that he is of himself, and, accordingly, call him the only beginning; but when we denote the relation which he bears to the Father, we correctly make the Father the beginning of the Son. Augustine’s fifth book on the Trinity is wholly devoted to the explanation of this subject. But it is far safer to rest contented with the relation as taught by him, than get bewildered in vain speculation by subtle prying into a sublime mystery.
 Bernard, De Consider. lib. v. “Cum dico unum, non me trinitatis turbat numerus, qui essentiam non multiplicat, non variat, nec partitur. Rurum, quum, dico tria, non me arguit intuitus unitatis, quia illa quaecunque tria, seu illos tres, nec in confusionem eogit, nec in singularitatem redigit” – See also Bernard, Serm. 71, in Cantica.